Due to their high functionality, 2-methylene alkanals are valuable intermediates in the industrial organic chemistry. By selective hydrogenation of the double bond, 2-methylalkanals can be obtained which play an important role in the fragrance industry. A further important reaction of this class of substances is obtained, for example, by oxidation of the aldehyde function into then unsaturated carboxylic acids, which are used on a large scale for the production of plastics, lubricating oils or textile auxiliary agents.
It is known to produce 2-methylene alkanals by reacting unbranched n-aldehydes with formaldehyde. The reaction is carried out in the presence of secondary amines in the form of a Mannich condensation reaction, wherein at first a Mannich base is formed under dehydration
from which 2-methylene alkanals are ultimately obtained by separation of the secondary amine:
(Methoden der Organischen Chemie, Houben Weyl, Georg Thieme Verlag, 4th edition, 1954, volume VII, part 1, pages 93-94).
The condensation reaction can in this case, as such, be carried out only with the classes of substances listed in the abovementioned reaction schemes or else can be catalyzed by the addition of acids or further bases.
DE 2855 506 A1 discloses a possibility for reacting alkanals only in the presence of catalytic amounts of secondary amines.
WO199320034 A1 describes the aldolization of C3 to C10 aldehydes in a continuously operated agitated vessel with subsequent distillation. Substituted acroleins are produced, wherein hydroxides or carbonates are used as additional catalysts.
An acid-base-catalyzed production method is disclosed, for example, in DE3744212 A1. In the process described therein, a C5 alkanal mixture obtained during the hydroformylation of isomeric butenes, is reacted during a reactive distillation with an aqueous formaldehyde solution in the presence of a secondary amine and a mono-, di- or polycarboxylic acid.
Just as diverse as the selection of the possible reaction conditions are also the operation modes and types of reactors which can be used for the production. For example, both batch and continuous processes are found in the patent literature, wherein the reaction is carried out, inter alia, in agitated vessels, agitated vessel cascades and also tube reactors. An example of an optionally continuous or batch reaction in an agitated vessel is given in the abovementioned DE 2855 506 A1. A continuous reaction in a tube reactor is described, for example, in DE 199 57 522 A1.
For the reaction of the employed aldehydes with formaldehyde in the presence of a catalyst, agitated vessels are typically used to ensure an intensive mixing of the organic and the aqueous phase. However, the reaction in agitated vessels is disadvantageous, since the discontinuous reaction control, in addition to heating and cooling processes, also exhibits a non-uniform amount of reaction heat, which can lead to suboptimal reaction results. In addition, the operation of mechanically moving parts is typically maintenance intensive and susceptible to repairs. On the other hand, a continuous reaction control within an agitated vessel or an agitated vessel cascade appears to be problematic due to the liquid multi-phase system because an undesirable one-sided discharge of one phase can lead to an enrichment of the other phase and thus leads to disadvantages by fluctuations of the average dwell time with respect to the conversion rate, selectivity and space-time yield.